1. Field of the Invention
The present invention is related to an adjusting device for the stroke position of an injection molding machine plasticizing screw when moving axially, suitable in particular for use with a pre-plasticization type injection molding machine separately equipped with a plasticizing part and an injection part, and more particularly to an adjusting device for adjusting a plasticizing screw for a retreat position.
2. Description of the Prior Art
In general there are two types of plasticizing screw equipped injection molding machines, a screw in-line and screw pre-plasticization.
The screw in-line injection molding machine melts and mixes resin with the screw rotating while retreating to weigh melt resin. The screw advances to inject such weighed melt resin into a metallic mold thereafter.
The screw pre-plasticization injection molding machine is furnished separately with a plasticizing part having a plasticizing screw and an injection part having an injection plunger. The plasticizing part rotates the screw to melt and mix resin and forward the resin to the injection part for weighing. The injection plunger injects the melt resin into a metallic mold.
The applicant lately proposed a screw pre-plasticization injection molding machine as shown in the patent laid-open No. 3-97518 that is provided with a hydraulic actuator to move a kneading screw axially in addition to a rotary (electric or hydraulic) motor to turn the kneading screw. The screw retreats in plasticizing and weighing to form an opening between the tip of the screw and a path to send melt resin to the injection chamber of an injection part, while during injecting the path is closed by the tip of the screw being maintained at the forward position. The machine is thereby able to prevent melt resin from flowing backward to the plasticizing chamber when injecting without using a check valve.
As shown in FIG. 5, a screw driving unit of an injection molding machine is such that a link shaft 2 is connected to the rear end of a screw 1, the link shaft 2 also being connected to the output shaft 5a of a rotary motor 5 via a spline 3. The link shaft 2 is rotatively supported by a piston member 7 by way of bearings 6, 6. The piston member 7 and a casing 9 together form a hydraulic chamber 10. There is formed a spline 1a at the rear end of the screw 1 to be spline connected with the link shaft 2. There is provided a position adjusting screw unit 4 in the center of the link shaft 2. The position adjusting screw unit 4 is equipped with an adjusting screw part 4a and a lock nut screw part 4b. The adjusting screw part 4a is screwed into a female screw part 2a of the link shaft 2, adjusting a projection quantity L, and a lock nut 8 is fastened to the screw part 4b at a suitable position to be secured to the link shaft 2.
Rotation of the rotary motor is thus transmitted to the screw 1 by way of the link shaft 2 at the time of plasticizing while the screw melts and kneads resin before it presses out melt resin to the injection chamber. Reaction force from such melt resin then acts on the screw 1 in a direction as indicated by an arrow A. The reaction force is received by the casing of the hydraulic chamber so as to cause draining by way of the position adjusting screw unit 4, link shaft 2, bearings 6, 6 and piston member 7.
The rotary motor 5 stops rotating in the injection process and pressurized oil is supplied to the hydraulic chamber 10 by way of an oil path 10a to move the piston member 7 axially and forward the screw 1 in a direction as indicated by an arrow B by way of the bearings 6, 6, link shaft 2 and position adjusting screw unit 4 so as to enable the tip of the screw 1 to keep the path shut to the injection chamber.
When the path leading to the injection chamber is closed with the tip of the screw advanced, resin at the tip of the screw flows into the injection chamber while part of the resin flows backward. It depends greatly on the fluidity or the viscosity of resin in the plasticization chamber how much resin flows backward. If the screw advances a lot, therefore, an error will be caused in weighing. It is desirable, accordingly, that the tip of the screw is retained in a certain position as near to the plasticization casing tip as possible to the extent that melt resin may be sent to the injection chamber without trouble at the time of plasticizing.
The foregoing screw driving unit has the position adjusting screw unit 4 rotating in relation to the link shaft 2 thereby adjusting the projection quantity L to allow the screw to be adjusted for the retreat position, with the tip of the screw unit 4 being brought into contact with the rear end of the screw 1. The screw 1 has a resin reaction force applied in a direction as indicated by an arrow A at the time of plasticizing. As a result, the rear end of the screw 1 comes into contact with the tip of the screw unit 4, which has been adjusted in position in relation to the piston member 7 and the link shaft 2 under the restriction of movement by the hydraulic chamber 10 in a direction as indicated by an arrow A. With this, the tip of the screw 1 is maintained as intended.
Now, referring to the adjusting mechanism based on the position adjusting screw unit 4, it is necessary to remove the rotary motor 5 and loosen the lock nut 8 using a special jig and turn the adjusting screw unit 4 to provide a rotation adjustment, because the screw unit 4 is situated in the center of the link shaft 2.
In addition, the adjusting screw unit 4 is situated inside, it is thereby not possible to provide a direct visual check of the amount of travel using a scale. Instead, it is required to depend on the rotational frequency and touch of the jig for the adjustment of the travel amount. It is difficult therefore, to measure the quantity of adjustment numerically or limit in terms of display. Not only is skill required, but it is difficult to provide accurate and regular positional adjustment.
As mentioned above, as the screw 1 closes the path to the injection chamber with a slight stroke of not more than 1 mm, it is necessary to adjust the screw 1 in position in an accurate manner. Nonetheless, accurate and regular adjustment is extremely difficult.
Furthermore, the position adjusting screw unit 4 is arranged in the center of the link shaft 2. Accordingly, this makes the diameter of the link shaft 2 larger, and the piston member 7 and the casing larger, too. This is an obstacle to downsizing the screw driving unit.